In the field of thermal insulation, vacuum insulation panels (VIPs) manufactured by enveloping a porous core material—for example, compacted fumed silica (Aerosil), fibre mats or open-cell foams—with a gas-tight sheet material and then carrying out evacuation are known. These panels permit excellent thermal insulation (thermal conductivities<3.5*10−3 W*m−1*K−1, determined in accordance with DIN 52 612, at 10° C., are realisable), but the insulating effect suffers markedly if the gas-tight sheet is damaged. These panels, consequently, have to be produced with particular desired dimensions and installed in a protected fashion (http://www.va-q-tec.com/).
The core material used has a substantial influence over the properties of a vacuum insulation panel. On one hand, the core material—as well as the gas and its residual pressure in the evacuated panel—determines the thermal conductivity and hence the heat insulation performance, and on the other hand, the mechanical properties such as compressive strength, fracture sensitivity and dimensional stability are substantially dependent on the core material.
The influence of the core material over the thermal conductivity can be subdivided into two mechanisms. The first is a direct contribution to thermal conductivity by thermal conduction in the solid phase of the core material. The second and more important mechanism is an indirect contribution by influence on the gas-phase thermal conductivity: if the core material possesses a very finely structured pore system, the gas-phase thermal conductivity—particularly at low gas pressures—may fall below the value anticipated for the gas composition. This effect is called the Knudsen effect. The Knudsen effect occurs when the free path length of the gas molecules is greater than the diameter of the pores in which the gas is located. Collisions of the gas molecules with the pore walls then become more probable than collisions of the gas molecules with one another. This may proceed to an extent such that collisions of the gas molecules with one another are suppressed entirely. Without collisions, there is no transfer of thermal energy, and gas-phase thermal conduction is switched off. Consequently, the smaller the average pore diameter of a core material in VIPs, the more efficient that core material. Small pore diameters cause the Knudsen effect to set in even at relatively high gas pressures, and the pressure does not need to be lowered to the same extent in order to suppress entirely the gas-phase thermal conductivity. A comprehensive discussion of these relationships, including measurements of the relationship between pressure and thermal conductivity for various core materials, is found at www.ecbcs.org/docs/Annex—39_Report_Subtask-A.pdf.
These measurements demonstrate that silica, more particularly fumed silica compacted to form boards/shapes, constitutes a particularly advantageous core material for vacuum insulation panels. The extraordinarily fine structuring of these powders results in a pronounced Knudsen effect. One disadvantage of using silica as a core material is the bad mechanical properties of this core material. Pressings made from compacted powders are naturally pressure-sensitive and fracture easily.
The open-cell polymer foams that are also part of the prior art generally have substantially better mechanical properties and can be brought by cutting into any desired form. However, because of the much higher pore diameter, such foams require a very low residual pressure in order to achieve the same insulating properties as panels with cores made from compacted silica.